Filter processes can essentially be divided first into a so-called cake filtration, deep filtration and surface filtration. While in cake filtration the filtration is performed by a filter cake formed on a relatively course substrate and in deep filtration the chief filtration effect takes place mechanically by adsorption within a filter medium, in the surface filtration the principle filtration effect takes place mechanically by separation, for example of solid particles on the surface of a filter medium, for example a filter element.
The filter element in accordance with this invention concerns surface filtration, in which the flow into the filter takes place either essentially perpendicular to the filter surface (so-called “static” or “dead end” filtration) or essentially parallel to the filter surface (so-called “cross flow filtration”).
In static filtration the retentate (the retained substances) forms a so-called filter cake, in which deep filtration increasingly takes place and which lowers the filter throughput over time. The formation of the cake is for the most part counteracted by a parallel flow over the surface of the filter medium and/or back-flushing through the filter medium.
Membrane filters in particular are suitable for surface filtration. The membrane filters that are most often used today have, for example, polymer membranes (for example, polyester, PP polyester, PVDF=polyvinylidene fluoride, etc.) or ceramide membranes (for example zirconium oxide, SiC, Si3N4, Al2O3, etc.). However, such membrane systems have numerous disadvantages. For instance, the distribution of the “pore diameter” is relatively broad in them, due to which the sharpness of separation of the membranes is poor. Substances that are really intended to be retained can then pass through the membrane. In the case of ceramic membranes one additionally runs up against the problem of the relatively low throughput, since these membranes have relatively long “pores” (in comparison with the “pore diameters”; thus more precisely speaking: channels) with high resistance to flow. Moreover, such membrane filters are limited with regard to chemical stability and temperature stability. With some of the said membrane systems there is also the problem of light cake formation (even in cross flow operation) because of the relatively uneven or rough membrane surface. Moreover, some of the said membrane filters are limited with regard to the maximum difference of pressures across the membrane (and thus with regard to an increase of the throughput by increasing the pressure differential).
Membrane filters based on Si or SiO2, Si3N4, etc. that are produced by etching or lithographing methods have been proposed in recent years.
One example is described in U.S. Pat. No. 5,543,046. This patent describes a method for producing an inorganic membrane that is applied by, for example, CV or sputtering methods to a macroporous carrier with a “flattening layer” initially arranged in between them. In an alternative embodiment a mechanical polishing is also mentioned instead of the “flattening layer.” The “flattening layer” is removed after the formation of pores.
Another example is described in U.S. Pat. No. 5,753,014. This patent describes a membrane filter and a method for producing it, in which the macroporous carrier can also consist of an inorganic material such as Si, SiC, Al2O3, etc. The membrane can also consist of polytetrafluoroethylene (PTFE), Si, C, [sic; SiC], Si3N4, SiO, Al2O3, a metal or steel, for example. In this method as well the pores are etched in the membrane layer by techniques that have long been known, for example from the semiconductor industry. After the pores have been formed the membrane is exposed by complete etching of the back side of the carrier layer. In an alternative embodiment the carrier structure can also be formed before producing the membrane. To reduce the tensile stress between the membrane layer and the carrier and for better bonding of them, an intermediate layer such as borax, chromium, nickel, etc., may be used. This patent also describes a pore filler material such as polysilicon, aluminum, etc., that must again be removed at the end of the process. In one embodiment a polyamide layer is structured as a masking layer for the membrane layer by means of a printing method (“imprint” and “liftoff” techniques) with the help of a printing form or in another embodiment the structured polyamide layer itself is used as the membrane layer.
In the case of U.S. Pat. No. 5,139,624 the pores are produced by wet chemical means.
In general one should note that filter elements made of at least two layers (a carrier layer and a membrane layer) have the problem that the coating methods mostly produce chiefly or completely amorphous layers, which is disadvantageous for mechanical strength.
Si3N4 is a material that is currently often used as the membrane layer. The prior art, however, shows that at present it is difficult to produce an Si3N4 layer with internal crystalline structure that goes beyond larger crystal nuclei, at temperatures under about 1400° C. The current art is at the laboratory and experimental level. The carrier structures of the filter elements mostly consist of Si, whose melting point is 1420° C. The heating/annealing of Si3N4 that is needed to produce a high crystalline fraction therefore would damage or even destroy the carrier structure.
The production of very thin membranes (<1 μm) with pore diameters <1 μm that nevertheless are stable with respect to relatively high pressure (>1 bar) with the currently known methods is difficult and has a high reject level. The limitation with respect to the ability to withstand pressure, which also is connected with the relative porosity and membrane thickness, makes filtration with high throughput expensive.